1. Field of the Invention
The invention relates generally to the deposition of materials and more specifically to the creation by laser deposition of composite layers of non-electrically-conducting polymers and electrically conducting particles.
2. Description of the Related Art
A chemiresistor is a chemical sensor made of a material that has an electrical resistance that varies in response to the presence or absence of an analyte. Typically, a chemiresistor comprises a composite coating of a non-conducting material (such as a chemoselective polymer) and a conducting material (such as carbon or metallic nanoparticles). The conducting material is dispersed throughout the non-conducting material with a sufficient density so that in the absence of an analyte, the chemiresistor conducts electricity. When the chemiresistor is exposed to an analyte, the analyte is absorbed by the non-conducting material, causing the composite coating to swell and causing the conducting particles to move farther apart from each other, thereby creating a measurable increase in electrical resistance through the coating.
Previously, chemiresistors have been made using conventional coating deposition techniques, such as spin coating, drop casting, or spray coating. These are xe2x80x9cwetxe2x80x9d techniques, meaning that they are processes that involve wetting a substrate surface with a mixture of the composite coating material and a solvent that volatilizes only after the deposit has been formed. For reasons discussed below, each of these techniques has the disadvantage that after the solvent evaporates, the resulting coatings tend to be non-uniform, lacking in homogeneity and imprecisely located. All of these factors are critical for the optimum performance of the chemiresistor.
With spin coating and aerosol spray coating, relatively large surface areas can be coated. However, it is difficult to achieve precise or accurate control of coating thickness with high reproducibility because of the difficulty in duplicating variables such as the cleanliness of the substrate, the viscosity of the solution, temperature conditions, the spin rate of the substrate and the delivery rate of the solution. Moreover, spin coating and aerosol spray coating are not well suited for coating discrete, micron-sized substrate areas because of the difficulty of using masking with these techniques.
Drop casting, which involves delivering a material to a substrate through a syringe using an X-Y controlled stage or X-Y controlled syringe, allows for the coating of the discrete, micron-sized substrate areas through the precise placement of single droplets of the solution on the substrate. However, this technique provides inadequate control over the physical parameters of the deposit. Once a drop lands on the substrate surface, the final resting place of the polymer-conductor material depends on the wetting of the solvent to the surface, surface tension of the solution, how clean the surface is, the viscosity of the solution, both at the start and at all stages as the solvent evaporates, the evaporation rate of the solvent, the temperature of both the solution droplet and the substrate, the vapor pressure of the solvent, and the colligative properties of the dissolved materials that lower the vapor pressure of the solvent. Because the placement of matrix material is controlled by the movement and evaporation of solvent molecules on the substrate surface, there is little control over the shape of the resulting polymer-conductor film. Most deposits made using the drop casting technique are rounded in shape when viewed from directly above. This results from the initial spherical shape of the droplet that is deposited on the surface. There are many other shapes possible that depend heavily on the degree of surface contamination. Substrate surface areas that are relatively clean and are significantly wettable by the solvent solution (as indicated by small contact angle) result in significant spreading of the solution, and hence, a spreading of the polymer-conductor material. In contrast, substrate areas that are contaminated and are not significantly wettable by the solvent solution (as indicated by large contact angle) result in a more localized concentration of the solution and thicker films of polymer-conductor material.
An additional disadvantage of the three xe2x80x9cwetxe2x80x9d techniques described above is that as the solvent evaporates from a deposit, an increasingly concentrated solution of the composite material is formed, which may result in some degree of phase separation of the conducting and non-conducting material through aggregate formation. As a result, the conductive material would not be uniformly distributed through the polymeric film when all the solvent evaporated. Areas of the polymer coating may not have the required quantity or uniform distribution of conductive material to allow electrical conductance. The resulting performance of a sensor fabricated in this fashion would be significantly impaired. If, in order to compensate for this deficiency, an increased loading of conductive material is used, the physico-chemical properties of the coating could be altered. For example, if a carbonaceous material is used as the conductive material, increasing the carbon loading causes the solubility properties of the coating to increasingly reflect the non-polar adsorptive properties of the carbon (see, for example, xe2x80x9cIntegrated Chemiresistor and Work Function Microsensor Array with Carbon Black/Polymer Composite Materialsxe2x80x9d, K. Domansky, V. S. Zapf, J. Grate, A. J. Ricco, W. G. Yelton, and J. Janata, Proc. Solid-State Sensor and Actuator Workshop, Hilton Head Islan, S.C., Jun. 8-11, 1998, pp 187-190, incorporated herein by reference.) An increase in non-polar adsorptive properties is undesirable if the polymer properties have been tailored towards sorbing polar analytes.
Accordingly, it is an object of the present invention to provide a coating of a composite material of a non-electrically-conductive polymer and electrically conducting particles wherein the electrically conducting particles are dispersed homogeneously throughout the non-electrically-conductive polymer.
It is a further object of the present invention to provide a method of making a composite coating of a non-electrically-conductive polymer and electrically conducting particles wherein the thickness, uniformity, homogeneity, location and surface coverage of the coating are precisely controlled.
These and other objects of the invention are achieved by a method of forming a layer of a composite material of a non-electrically-conductive polymer and electrically conducting particles wherein the layer is formed by laser deposition. The creation of the layer can be done either by pulsed laser deposition, matrix assisted pulsed laser evaporation (MAPLE) or a direct write form of matrix assisted pulsed laser evaporation (MAPLE-DW).